This is a case where specific beats general. Book of Ancient Secrets does exactly what it says, which is that it allows you to:
[A]dd other ritual Spells to your Book of Shadows. When you find such a spell, you can add it to the book if the spell’s level is equal to or less than half your warlock level (rounded up)
3/2 = 1.5 ~= 2. You can transcribe any and all rituals your warlock finds that are <= spell level 2. Detect Magic is level 1 and a ritual. 1 < 2, so you can transcribe it.
No. Eyes of the Rune Keeper Does Not Read Spells.
tl;dr "Generally, you would not know anything about a scroll, even if you can read every language. Scrolls don't use a language." – Daniel Zastoupil
In short, the words (if any) could be understood in a literal sense, but that doesn't get the meaning nor substance of the spell. Spells, such as those on scrolls, don't have linguistic meaning. They are a cipher.
A spell scroll bears the words of a single spell, written in a mystical cipher.
Ciphers are (generally) not pronounceable.
An example simple cipher using rot-13 of "i am a cipher" has the cipher text:
v nz n pvcure
There is no meaningful pronunciation once the letters are rotated. Other ciphers such as digraph substitution are even less intelligible.
No Linguistic Meaning.
In the case a cipher is chosen such that the cipher text is pronounceable, they wouldn't necessarily be words with a linguistic meaning. For example, while pronounceable, the following cipher text is gibberish:
malveS IBEr OLma ScriANg BOAGern EYmpGREn ToR nUmialaM
Pronounceable Codes are Cryptolects
While the warlock would be able to technically read and pronounce the words, they would be unintelligible.
The ability does not now endow the possessor with the ability to
understand codes. Even if the spell was not strictly a cipher, but a cryptolect analogous to thieve's cant or irish traveller's language, it would still be unintelligible.
As Crawford says in this tweet:
Eyes of the Rune Keeper lets you read all writing. That doesn't mean you understand a secret code being delivered by that writing. For example, you might read, "Sunset Dog Potato," and have no idea that's code for something. #DnD
Reading & Casting Clarification by JC
Jeremy Crawford answered a related question about casting in this thread.
JC: Eyes of the Rune Keeper lets you read any form of writing, including the linguistic meaning of a rune, if any. #DnD
@ZerounTheQuick: So does that mean they could read spell scrolls and cast with the normal DC for spell scroll castings?!
JC: Nope.
Best Answer
No; reading the scroll is separate from being able to cast the spell.
The description of the spell scroll magic item begins (emphasis mine):
As you note, the Eyes of the Rune Keeper eldritch invocation lets you read all writing - but that only tells you the linguistic meaning of the writing. It doesn't grant you any special ability to cast a spell that you couldn't otherwise cast. Being able to read the writing on the scroll is not the same as being able to cast the spell, and you can't cast a non-warlock spell from a scroll without having access to that class's spell list (e.g. by multiclassing).
Rules designer Jeremy Crawford unofficially confirmed this ruling in this series of tweets from March 2016 (the first part of which is also stated in the Sage Advice Compendium):
While you might be able to understand the literal meaning, if any, of the words written in the "mystical cipher" on the spell scroll, that doesn't grant you the ability to cast the spell on it. For instance a scroll of knock might read "open sesame", but you still won't be able to read the spell scroll without multiclassing into bard/sorcerer/wizard, or being able to add it to your spell list some other way.